The present invention relates to a device for positioning two movable bodies which are driven independently of each other and, more particularly, to a device for positioning two rotary bodies. Further, the present invention is concerned with an image forming apparatus of the type having photoconduction means and transfer means which are each implemented with a drum or belt and are driven independently of each other to be individually rotatable at variable speeds, and in which the rotation of the transfer drum relative to that of the photoconductive drum is accelerated during the interval between consecutive image transfers in matching relation to a size of paper sheets so as to increase the recording speed, particularly in a color copier or like color image forming apparatus capable of reducing a period of time necessary for copying.
It has been customary to adopt an arrangement wherein a color original document is repetitively scanned by optics which includes a plurality of color separating filters while, at the same time, exposure by a plurality of separated color components are sequentially effected. The resulting latent image formed on a photoconductive drum or other photoconduction means, are individually developed by toner or complementary colors which are supplied by a developing device, and the resulting toner images are sequentially transferred to a paper sheet which is clamped on the transfer drum, or other transfer means, which is in turn held in contact with the photoconductive drum. The photoconductive drum and the transfer drum are interconnected by, for example, gears so as to be driven together at a constant speed. The circumferential dimension of the transfer drum is at least approximately the same as the length of the largest paper size usable with the copier, e.g. size A3. Hence, in the case that a toner image is transferred to a paper sheet of size A3 by way of example, the transfer drum has to be wastefully rotated by one rotation while the optics scans back, resulting in the transfer taking approximately twice as long a period of time as in the case of transfer of a toner image to paper sheets of sized B4 and A4.
A color copier designed to eliminate the above drawback is disclosed in, for example, Japanese Laid-Open Patent Publication (Kokai) No. 60-218673 (Fuji Xerox). In the color copier there disclosed, the optics scans and scans back on a color component basis in matching relation to a paper size, and the rotation speed of a transfer drum is varied during the interval between consecutive transfers and based on the paper size so that the leading end of the next toner image of a particular color and that of a paper sheet may coincide with each other at a predetermined transfer position. That is, a particular copying speed is set up which matched with a particular paper size. To so vary the speed of the transfer drum, the transfer drum and the photoconductive drum are driven by individual drive sources (servo motors). The transfer and photoconductive drums are dimensioned substantially equal to each other in their axial direction and held in direct contact with each other under a predetermined pressure. Although not clearly shown or described in said laid-open publication, a transfer drum of the above-described type of prior art color copier has a rotatable drum which is notched along its axis to form an intermediate opening. The intermediate opening is delimited at opposite axial ends thereof by end portions of the drum. A dielectric sheet such as a dielectric film or an electrostatic screen is wrapped around the end portions of the drum. With this configuration, the opposite end portions of the transfer drum make contact with the photoconductive surface of the photoconductive drum with the intermediary of the dielectric sheet.
With the above construction, however, it is almost impracticable to set up a copying time appropriate for a particular paper size by varying the rotation speed of the transfer drum relative to that of photoconductive drum, as stated earlier. Specifically, since the photoconductive and transfer drums press against each other via the dielectric sheet at opposite ends thereof, the frictional force acting between the two drums when the speed of the transfer drum is changed is too great to permit smooth slippage of the drums. Any excessive force would result in vibrations and, therefore, in blurring, jitter and various other causes of incomplete image reproduction.
Further, even if the speed of the transfer drum is not changed, the dielectric sheet is pressed by a substantial transfer pressure by the opposite end portions of the transfer drum, which is rigid, making rolling contact with the photoconductive drum. This brings about some other problems as follows. Both the transfer and photoconductive drums have to be machined with considerable accuracy at the sacrifice of cost. Rolled by the high transfer pressure, the dielectric sheet is deformed at opposite end portions of the two drums resulting that a toner image is prevented from being uniformly transferred to a paper sheet. In addition, toner, paper dust and other particles are apt to deposite on the opposite end portions of the two drums, disturbing uniform transfer of a toner image.